Conductors over which digital signals are transmitted must be properly terminated in order to prevent overshoot, undershoot, and reflections. These effects, when caused by impedance mismatch, become more pronounced as the length of the conductor increases, and limit the rate at which data can be transmitted over a conductor. The conductor can be a trace on an integrated circuit, a trace on a board, or a wire in a cable. Furthermore, a conductor may include one or more such components. The impedance of both the source and load should be matched to the characteristic impedance of the conductor. Since the output impedance of the transmitter and the input impedance of the receiver generally differ from the characteristic impedance of a conductor that interconnects the transmitter and receiver, it is necessary to alter the existing impedance differently at the source and load ends of the conductor. When the conductor is used for bidirectional signal transmission, each end of a conductor is a source or a load at different times. A single impedance alteration does not provide the correct impedance matching for both transmission.
One known technique for solving impedance mismatch is disclosed in U.S. Pat. No. 4,748,426 which teaches permanently connecting one end of each of a pair of resistors to a wire in a cable for coupling a plurality of peripheral devices to each other. The other end of the resistors are connected through unidirectional switching devices to a positive voltage supply line and to logic ground, respectively. While this technique can be used to match the input impedance of the receiver to the characteristic impedance of the wire, it does not provide for matching the output impedance of a transmitter to the characteristic impedance of the cable wire. Furthermore, due to the resistors being permanently connected to the wire, this technique does not accommodate bidirectional signal transmission.
What is needed is an improved termination technique that matches the impedance of both a transmitter and receiver to the characteristic impedance of a conductor to which they are coupled during the time each is active.